A conventional inkjet printing system includes a printhead assembly, an ink supply which supplies liquid ink to the printhead assembly, and an electronic controller which controls the printhead assembly. The printhead assembly, commonly referred to as a print cartridge or pen, ejects ink drops through a plurality of orifices or nozzles and toward a print media, such as a sheet of paper, so as to print onto the print media Typically, the orifices are arranged in one or more arrays such that properly sequenced ejection of ink from the orifices causes characters or other images to be printed upon the print media as the printhead assembly and the print media are moved relative to each other.
To move the print media relative to the printhead assembly and route the print media through a print media path, the conventional inkjet printing system includes a print media transport assembly. Typically, the print media transport assembly includes one or more rollers or wheels each rotatably mounted for contacting the print media and routing the print media through the print media path. In order to route the print media under and through a print zone between the printhead assembly and the print media and hold the print media in position during printing, the print media transport assembly often includes a number of starwheels each formed with a plurality of radially spaced tips. As such, the starwheels are positioned in opposing relationship to and contact output drive rollers such that the print media is fed into engagement between the starwheels and the output drive rollers after the ink is deposited on the print media. Thus, the starwheels and the output drive rollers are positioned on an exit side of the print zone.
Unfortunately, as the starwheels contact the print media, the starwheels may pick up the newly deposited ink and redeposit the ink on the print media thereby causing tracking on the print media. This problem becomes worse as printing speeds increase since the time between deposit of the ink on the print media and contact of the print media by the starwheels is reduced. Thus, the newly deposited ink may not have sufficient time before contact by the starwheels.
In addition, since the tips of the starwheels contact the opposing output drive rollers, surface materials of the starwheels and the output drive rollers must be compatible to prevent excess wear of the tips of the starwheels and/or the surface of the output drive rollers. For example, the starwheels are often formed of stainless steel or plastic and the output drive rollers are often formed of plastic or rubber. Forming the output drive rollers of plastic or rubber, however, does not facilitate the most accurate routing of the print media during printing thereby leading to image quality defects. Also, a bottom print margin of the print media must be sufficient to ensure that the print media is held in position on an entry side of the print zone by other rollers or wheels of the print media transport assembly other than the starwheels and the output drive rollers. Consequently, a size of the bottom print margin which is defined as a distance between rollers on the entry side of the print zone and the print zone itself limits how close printing can occur to the bottom the page. Such a limit is undesirable, for example, for duplex printing where a bottom print margin on a second side of the print media dictates the actual top print margin for that side of the print media although equal top and bottom print margins for both sides of the print media are preferred.
Accordingly, a need exists for accommodating faster printing speeds and reducing a size of a bottom print margin while using a starwheel to route a print media through a printer. In particular, a need exists for controlling actuation of a starwheel of a print media transport assembly so as to minimize tracking on the print media by the starwheel as well as minimize wear between the starwheel and an output drive roller such that the output drive roller may be formed of a suitable material to enable more accurate routing of the print media during printing.